While the classic pulsed plasma thruster (hereinafter “PPT’) propulsion system technology is mature, it has historically been limited by its high mass and small propellant load to precision pointing and small delta-V applications. The PPT has a technology readiness level (TRL) of 9, having flown on several spacecraft beginning with the Soviet Zond 2 mission in 1964, accumulating over 30 thruster years in space through 1991. The LES 8/9 PPT was not flown but demonstrated 34×106 pulses during development and flight qualification. More recently (2000-2017), the PPT was employed for pitch control on the Earth Observing 1 (EO-1) spacecraft. The principal use of these PPTs has been for attitude control and precision orbital adjustments including stationkeeping, but not for primary propulsion applications such as orbit change and de-orbiting. Extensive flight experience demonstrates that technical risk areas such as pulse electromagnetic interference, exhaust condensation and system life have been successfully mitigated, supporting a TRL 9 rating.
An attempt at higher impulse PPT applications was an Air Force Rocket Propulsion Laboratory/Fairchild Industries program, which concluded in 1977; this project was devoted to developing the PPT for stationkeeping of 500 kg-class satellites, producing a one-millipound (4.4 mN) PPT with an impulse capability of 166,000 N-s from 10.6 kg of PTFE (Teflon™) propellant. The twin rectangular propellant bars were stored as opposed helices, and the pillbox-shaped thruster envelope had a volume of ˜85 liters, with a total estimated system mass, including 10 kg of high voltage capacitors, of 24 kg. The self-field {right arrow over (j)}×{right arrow over (B)} device generated thrust between plane parallel electrodes through a side exhaust nozzle from 450 J pulses at 0.20 Hz, at a mean thruster power of 90 W. System specific mass was α=210 kg/kW. The PPU mass was 2.4 kg, and the PPU specific mass was high at ˜15 kg/kW. A question remains as to the accuracy of the specific impulse, as was claimed as 2200 seconds, and did not include eroded electrode mass in the calculation. The one-millipound thruster demonstrated that the Teflon PPT can generate very high total impulse, with a volumetric impulse of 2700 N-s/liter, but did not demonstrate low specific mass (kg/kW).
Historically, pulsed plasma systems have targeted small delta-V applications such as attitude control. With Applicant's Fiber-fed Pulsed Plasma Thruster (hereinafter “FPPT”) and its innovative propellant feed and storage system, the FPPT is projected to outperform previous state of the art PPT systems, as well as newer technologies, With an anticipated >5,000 N-s total impulse from a 1U system, and a 1U mass of <1.5 kg, 100 s of km orbit transfers and inclination changes of tens of degrees are now available to smaller satellites. The intrinsic safety of FPPT and its inert, unpressurized PTFE propellant position it as a prime candidate for secondary payload missions where costs and logistics are dominated by range safety concerns. The solid propellant has no handling, storage, or operational restrictions. The ease of handling and storage for the solid propellant can extend operation to planetary missions with no additional monitoring or controls. FPPT system unit costs are anticipated to be significantly below competing liquid or gas-fed CubeSat propulsion systems.
Specific goals stated in NASA's 2015 Roadmap In-Space Propulsion Technologies Technical Areas 2.1.1, Chemical Propulsion, and 2.1.7, Micropropulsion, are “Enhance current missions and open up new mission opportunities through improvements in performance, manufacturability, durability, and cost”, “Develop engines that operate on non-toxic storable propellants”, and “Develop compact and lightweight systems with high precision control capability.” Applicant's FPPT propulsion system responds directly to these goals with a focus on high total impulse performance with cost reduction through common commercial-off-the-shelf (COTS) materials of construction.
Commercial interest in very small satellites continues to grow in the 1-500 kg satellite sector. Moving forward, it is more important than ever that these satellites have access to propulsion systems to extend their asset time on orbit. The FPPT system offers CubeSats and larger small satellites a significant propulsion capability with high impulse per unit volume. The Teflon propellant has no handling, storage, and operational restrictions, FPPT will require no safety equipment for storage, transportation, integration, and testing, and place no demanding requirements on the launch provider, making it an ideal low-cost solution for industry, research, and academic small-satellite propulsion needs.
Potential CubeSat and nanosatellites missions with FPPT include low Earth orbit raising and/or deorbiting. FPPT would improve mission affordability for multiple CubeSats, since several CubeSats with FPPT could be launched from a single low-cost booster and maneuvered to other orbits, then later de-orbited. The FPPT thruster will provide a compact, low mass, non-hazardous propulsion technology solution that will be made available in a family of sizes by changing the propellant spool volume to meet the differing needs of users in NASA, DOD, industry, and universities for CubeSat and small-satellite missions.